Noise

Quiet places

Closed building blocks can act as a barrier against environmental noise from outside, with green courtyards and squares in the middle to create quieter places, even in a busy city centre. Taller plants that surround buildings can have a sound-absorbing effect.

Greenery dampens noise

Sound is measured in decibels (dB). A person can easily perceive a difference of 3 dB in sound volume. If the noise is reduced by 5 dB, it is perceived as twice as loud. Green roofs contribute to a more acceptable noise level, both inside and outside buildings. Depending on the thickness of the roof used, they can reduce noise pollution by up to 46 dB. Vegetated facades and walls absorb 2 to 3.5 dB of street noise. Green façades prevent sound from reverberating between tall buildings.

Individual trees can reduce noise by up to 2 dB. A strip of 15 to 30 metres wide of several dense rows of trees with dense crowns can reduce traffic noise by 6 to 10 dB respectively at 50 meters from the road.

Relatively quiet

In general, less than 40 dB is considered to be a low noise exposure. However, 60 dB can also be experienced as quiet places if greenery is present. In addition to the absolute noise level, the contrast with the environment plays a role. A place can also be perceived as quiet if it is clearly less noisy than its surroundings. For a clear contrast, a difference of at least 10 dB with the environment is recommended.

Tranquillity is an important and desirable quality in cities. It is essential to not fill all parks with activities and to keep opportunities to experience the peace and quiet. Research has shown that urban residents who live close to parks or other green areas experience less noise nuisance with the same level of actual  noise than urban residents without a park or other green area near their homes.

Planted noise barriers

Planted noise barriers with a large mass at their core can be an alternative to screens made of, for example, concrete. In addition to acting as a barrier, they also offer the possibility of wildlife solutions, a carbon sink, and particulate matter adsoption.

The green options may differ on the road or back of the screens.

Base layer

The soil also has an effect on reducing noise pollution and absorbing sound. A paved or frozen floor reflects a lot of sound. So does water. A loose, more vegetated base absorbs sound better.

According to research, trees and shrubs are able to reduce high-frequency noise in particular. Low tones are mainly muffled by the ground.

Seasons

In winter, reduction of noise pollution by deciduous plants will be very limited. There are no rustling leaves and fewer bird sounds that can mask the noise. Coniferous trees with densely spaced branches can dampen the sound to some extent throughout the year.

Attractive natural sounds

Greenery contributes to reducing the noise nuisance experienced. It produces pleasant sounds for us directly (by the rustling of trees and shrubs) or indirectly (by attracting chirping birds). The noise of traffic or industry is usually not drowned out by these sounds of nature. However, green sounds do make the noise less noticeable. It is important to pay attention to these proven effects of greenery on our perception of noise and the experience of noise nuisance.

The presence of water in the environment also unconsciously gives us a sense of safety and tranquillity. If we see greenery and hear water at the same time, our experience of peace and reduced stress are enhanced. A pond does not absorb sound itself, but it does reflect it. The pleasant sight of a pond does provide distraction. Splashing water from a fountain masks or distracts from urban background noise.

Less nuisance with green views

Research shows we pay less attention to annoying background noise when we have a visually pleasant green environment. High-quality greenery attracts people. The presence of people around us can strengthen our sense of security. A green environment with people feels more pleasant, so that noise is perceived as less annoying. When people are asked about their favourite quiet place in the neighbourhood, they often also mention things like cleanliness, pleasant smells and colours, and well-maintained places.

Noise from road traffic is perceived as less disturbing if the road traffic is not visible. Green can be used for screening to provide this benefit.

Implementation

Thickly planted noise barriers are perceived by local residents as more pleasant than noise barriers made of hard materials such as concrete, metal or glass. In addition, it may be easier to gain approval for permits and any subsidy for a green noise barrier. It is important that the plant selection and attention to the soil are included in the design in advance of implementation. It is important to not choose plants that are too small or that are too large for the site.

Take into account the orientation of the aspect (N-S-E-W), wind direction, and winter hardiness. Very high demands are placed on the range of climbing plants that are suitable for vegetation of noise barriers along motorways. Below is a brief overview.

The basic requirements are:

• Give more ornamental value than a bare screen
• Firm adhesion to screen or climbing support
• Do not cause damage to the screen
• Durable (at least 10-15 years of vitality)
• Should cope with little maintenance
• Resistant to drought and frost
• Not very susceptible to pests and diseases
• Resistant to exhaust fumes and to some extent to road salt

In addition, at some locations the following are desired:

• Fast and preferably dense vegetation
• Do not cause traffic disruption (by growing too wide)

© Hiemstra, Hops Climbing plants on noise barriers Road and Water Engineering, PPO 2004

In addition to the commonly used evergreen ivy (good for particulate matter and resistant to air pollution), other plants are also suitable. Use accents plants tailored to the environment. Think of wild rose or painted broom where the residents’ side and road side can differ.

Structural noise barriers along roads can be enhanced by greenery. Ivy is not always the best choice. On a light-coloured concrete screen or a south-facing screen, ivy adheres and climbs ineffectively.

Materials

Noise barriers can be made of different materials. For spider plants and supported climbers, the type of material is not an important factor. These can also be used on transparent materials made of glass or plastic. For self-adhering plants, the material surface is decisive. Rainwater on concrete can release a high pH value or toxic substances such as heavy metals. These are not good for climbing plants.